1. Field of the Invention
The present invention relates to an electric power steering system.
2. Description of the Related Art
In general, an electric power steering system (EPS) has a function of detecting the occurrence of an anomaly of a motor when the energization failure occurs in any of phases of the motor (any of U, V, and W phases) due to a break in a power supply line, a contact failure of a driver circuit, or the like. Examples of the energization failure include the break in a power line that connects between the driver circuit and a motor coil in each phase, and an open fault (permanent open fault) of each switching element that configures the driver circuit. Conventionally, when such the failure is detected, the configuration that stops the motor immediately and provides a fail-safe is commonly employed.
However, the stop of the motor as a drive source results in a certain increase of a steering effort that is required of a driver. Therefore, in recent years, a continuous power assist control is demanded even after the occurrence of the energization failure as described above.
For example, a pamphlet of International Publication No. WO 2005/91488 discloses the configuration that continuously runs the motor, when the energization failure occurs in only one phase, by using two phases other than the energization failure occurring phase, as energized phases. The above configuration is also able to generate uniform motor torque by setting a specified rotation angle corresponding to the energization failure occurring phase as an asymptote and flowing a phase current that varies in a secant curve (a curve based on sec θ=1/cos θ) pattern or a cosecant curve (a curve based on cosec θ=1/sin θ) pattern to each energized phase. Details of the method of performing a two-phase drive control that flows the phase current in the secant curve pattern or the cosecant curve pattern are disclosed in Japanese Patent Application Publication Nos. 2008-211909 (JP-A-2008-211909) and 2008-211910 (JP-A-2008-211910), for example.
In addition, Japanese Patent Application Publication No. 2004-10024 discloses the configuration that includes dual-system motor coils that are provided independently and multiple driver circuits that are capable of independently energizing each of the motor coil systems. When the above configuration is applied to the EPS, even if the energization failure occurs in one system, the continuous power assist control can be achieved through the power supply to the other system.
However, when the two phases other than the energization failure occurring phase are simply used as the energized phases, significantly large torque ripples arise (for example, see FIGS. 9 and 10 of JP-A-2008-211909). Although the uniform motor torque can be generated through two-phase energization in the secant curve pattern or the cosecant curve pattern, the phase current that can actually flow has a limit, and a drop in the motor torque that arises near the asymptote due to the execution of the current limit is unavoidable. Even if the motor coil and the driver circuit as well as a power supply line are configured to be the dual systems, as long as the configuration utilizes the motor torque of both systems, the power drop due to a stop of the energization of one system is unavoidable. As described above, any of the conventional techniques involve the decrease of steering feel in the continuous control after the occurrence of the energization failure, and in this regard, room for improvement remains.